It's super powerful and simple:

# This will be OOM-killed inside its own namespace
sudo systemd-run --scope -p MemoryMax=100M \
    unshare --fork --pid --net --mount-proc \
    python3 -c "import os; print(f'My PID: {os.getpid()}'); a = bytearray(200 * 1024 * 1024)"